Modern wireless telecommunications systems are evolving to provide high speed packet data services for users of mobile equipment. One example is an ability to provide internet access to a user of mobile equipment. One wireless system that is rapidly evolving in this direction is a Time Division, Multiple Access (TDMA) system known as the Global System for Mobile Communication (GSM), in particular enhanced versions of GSM known as GSM+, GPRS (General Packet Radio Services) and EGPRS (Enhanced General Packet Radio Services).
A new mode of operation, referred to as the Compact Mode (Compact), was specified for GSM Release 99 (R-99). Compact is a radio interface mode for inter-cell synchronized systems, wherein the mapping of control channels for up to four cells is done on the same carrier, and where control channel separation is achieved by transmitting control in different cells on different timeslots. The mapping of the control channels has been specified in a document entitled 3GPP TS 05.02. A Compact Packet Broadcast Control Channel (CPBCCH) is specified to be transmitted with constant RF output power on at least four radio blocks per multiframe on a serving time group.
The Compact Mode thus introduces a discontinuously transmitting broadcast control channel. The use of discontinuous transmission on the broadcast control channels, with a synchronous inter-cell wireless network, enables a GSM network deployment using only 1 MHz of bandwidth, as opposed to the currently specified 2.5 MHz of bandwidth that use continuously transmitting control channels. The new (discontinuously transmitting) control channel for the Compact Mode was specified for IS136+EGPRS systems so that current IS136 system operators could deploy a new EGPRS radio system with a minimum initial spectrum allocation.
As may be appreciated, the introduction of the discontinuously transmitting control channels requires modifications to be made to the existing implementations of mobile stations. These changes relate to the initial scanning for the discontinuously transmitting control channels, synchronization to the discontinuously transmitting control channels, and the performance of neighbor cell measurements using the discontinuously transmitting control channels.
One particular problem that is introduced by the adoption of the Compact Mode relates to the operation of frequency synthesizer of the mobile station. The frequency synthesizer is a programmable frequency generator used for tuning between the transmission and reception frequencies over bands of transmission and reception frequencies. It can be shown that the use of the Compact Mode, where the mobile station is required to make a neighbor cell Compact Mode channel measurement (when operating in a Traffic mode as opposed to an Idle mode) requires that the frequency synthesizer exhibit a maximum settling time of about 250 microseconds. Contrasting this requirement with the current GSM synthesizer settling times in the range of about 500 microseconds to about 600 microseconds, it can be appreciated that the synthesizer operational characteristics are much more stringent.
GSM 05.08, chapter 12.4.1.2 (Packet Transfer Mode) states that while in the packet transfer mode the mobile station must continuously monitor all Broadcast Control Channel (BCCH) or CPBCCH carriers as indicated by a BA (GPRS) list, as well as the broadcast carrier of the serving cell. In every TDMA frame possible, a received signal level measurement sample must be taken on at least one of the BCCH or CPBCCH carriers, as evenly distributed as possible amongst the neighbor cells. For CPBCCH carriers, only the TDMA frames where common control or broadcast blocks are transmitted are used for monitoring signal levels.
This implies that no matter what the traffic time slot allocation to a particular mobile station, the mobile station is expected to take at least one receive (RX) level sample from a neighbor cell signal in every TDMA frame. In order to comply with this requirement the mobile station requires a fast (250 microsecond) synthesizer. For example, when operating with multislot class 6 (i.e., a capability to receive three slots (3 RX) and transmit in another slot (1 TX) during a single frame, which in GSM contains 8 time slots), the mobile station's frequency synthesizer must exhibit a maximum settling time of 250 microseconds.
Reference in this regard can be had to FIG. 1, which shows two consecutive GSM frames, wherein the Compact Mode transmissions occur only during timeslots 1, 3, 5 and 7 (the even times lots 0, 2, 4 and 6 could be used instead). FIG. 1 also assumes the mobile station is operating multislot class 6. Since the mobile station must make 3 RX and 1 TX during a single frame, then the mobile station is capable of “seeing” a neighbor cell signal only in timeslot positions 5 and 7. However, since the mobile station must receive its first RX timeslot immediately following timeslot 7, due to synthesizer tuning and settling times, it is not desirable to attempt to make a neighbor cell measurement during timeslot 7. Instead, all of timeslot 7 should preferably be reserved for settling the frequency synthesizer in order to accurately receive the transmission in timeslot 0 of the next frame. As such, only timeslot 5 is available for making a neighbor measurement when using the illustrated timeslot traffic allocation. In this case the synthesizer settling time must begin immediately after the timeslot 4 TX has ended. The synthesizer must be settled within 250 microseconds in order to be able to adjust the automatic gain control (AGC) and take a 32-bit analog to digital converter (ADC) RX-level sample before the end of timeslot 5. More specifically, assuming that the AGC adjustment requires 7*8=56 GSM bit times, and the actual RX-level sampling requires 32 GSM bit times, what remains for the synthesizer settling time from a single 156 bit GSM burst (timeslot) is 156−56−32=68 GSM bit times, which corresponds to 250 microseconds.
The foregoing problem does not exist in the conventional GSM network, since all of the control channel carriers are constantly transmitting. As such, the mobile station may make neighbor cell measurements at any point in a TDMA frame. However, since the neighbor cells in the Compact Mode are transmitting only during every other timeslot (e.g., 1, 3, 5 and 7), the timing of neighbor cell measurements must be made quite precisely.